How to produce documents like factset blackline?
Financial information companies spend a very large amount of effort to do this kind of thing properly, and their models are generally proprietary, so the real answer is not something you'll be able to get on a public site.

Of course, you can achieve a very simple approximation with a literal diff on the texts, and a more sophisticated one by using out-of-the-box NLP or computer vision techniques, but you're probably not going to get close to the performance of factset or other competitors using those tools.